Microcapsulation technique is widely adopted for the purpose of, e.g., protection or controlling the rate of liberation to outside of a comminuted core or content material. Examples of the content material include: agricultured chemicals, medicines, toilet materials such as perfume, color-forming materials and adhesive.
Such microcapsules may assume commercial product forms of microcapsules alone or a liquid mixture with a diluting carrier. It is also widely practiced to formulate microcapsules into an aqueous suspension (or emulsion) liquid for spreading the microcapsules to be used after dilution with an appropriate amount of water, as desired, prior to the spreading, e.g., for microcapsules of agricultural chemicals, inclusive of insecticides, fungicides, herbicides, virucides and attractants.
Most important properties for such an aqueous microcapsule suspension liquid include that it is stable without causing sedimentation or separation of the microcapsules during storage and preservation, that even if some sedimentation of microcapsules is caused, the microcapsule suspension liquid or aqueous diluted suspension liquid thereof is capable of easy re-dispersion by a light degree of shaking, and that the microcapsule suspension liquid is easily diluted with water to provide a stable aqueous diluted suspension liquid.
Hitherto, such an aqueous microcapsule suspension liquid has been generally formed by adding materials, such as a thickening agent, a dispersing agent, an anti-freezing agent, an antiseptic agent and a specific gravity-adjusting agent. It has been known that the selection of a thickening agent has a great influence on the suspension stability of the resultant microcapsule suspension liquid. Examples of known thickening agents used for such purpose include: water-soluble synthetic polymers, such as polyethylene oxide, carboxymethylcellulose, and polyvinyl alcohol; water-soluble natural polymers, such as gum arabic, guar gum, locust bean gum, and sodium alginate; (hetero)polysaccharides formed by fomentation with microorganisms, such as xanthan gum, rhamsan gum and welan gum; and natural minerals, such as montmorillonite, and aluminum magnesium silicate. It is also known that polysaccharides formed by fermentation with microorganism as represented by xanthan gum formed by fermentation with a strain of microorganism of the genus Xanthomonas campestris particularly exhibit an excellent thickening effect and a relatively stable microcapsule suspension liquid (Japanese Laid-Open Patent Application (JP-A) 2-28813).
However, according to our study, it has been found that a microcapsule suspension liquid formed by using xanthan gum as a thickening agent is relatively stable but still causes some sedimentation or viscosity increase with time and results in a state, such as aggregation or caking, which makes difficult the use thereof by re-dispersion, after long-term of storage.